Three-dimensional printing, also known as additive manufacturing, is a process of making a three-dimensional solid object from a digital model of virtually any shape. Many three-dimensional printing technologies use an additive process in which successive layers of the part are built on top of previously deposited layers. Some of these technologies use inkjet printing, where one or more printheads eject successive layers of material. Three-dimensional printing is distinguishable from traditional object-forming techniques, which mostly rely on the removal of material from a work piece by a subtractive process, such as cutting or drilling.
In addition to forming three-dimensional objects with different shapes, some three-dimensional printers also form printed images on one or more surfaces of the object. The printed images are, for example, decorative or include printed images and text indicia that label or provide instructions for the use of the three-dimensional printed object. A three-dimensional object printer can be configured to eject drops of various types of ink to form the printed images, including both monochrome and multicolor images, using halftone printing techniques that are well known to the art.
Existing halftone printing techniques are unsuitable for many three-dimensional printing scenarios because the halftone printing process produces a layer of printed ink with a non-uniform thickness since regions of the printed image that receive ink drops are thicker than the regions of the image that remain blank. The printer cannot form additional layers of build material over the halftone printed image in a reliable manner since the halftone image does not provide layers with uniform thickness to support additional layers of build material in the object. Additionally, filling in the entire region with ink to form a uniform layer greatly reduces the accuracy of the halftone color patterns and greatly reduces the quality of printed images. Consequently, improvements to the operation of three-dimensional object printers to enable halftone printing and enable the formation of additional layers of material over halftone printed images would be beneficial.